1. Field of the Invention
The present invention relates to a driving device for driving a liquid crystal display (LCD) device, and more particularly, to a driving device for performing corresponding charge sharing according to a driving approach of the LCD.
2. Description of the Prior Art
The advantages of a liquid crystal display (LCD) include lighter weight, less electrical consumption, and less radiation contamination as compared to other conventional displays. Thus, LCD devices have been widely applied to various portable information products, such as notebooks, PDAs, etc. In an LCD device, incident light produces different polarization or refraction effects when the alignment of liquid crystal molecules is altered. The transmission of the incident light is affected by the liquid crystal molecules, and thus magnitude of the light emitting out of the liquid crystal molecules varies. The LCD device utilizes the characteristics of the liquid crystal molecules to control the corresponding light transmittance and produces gorgeous images according to different magnitudes of red, blue, and green light.
Please refer to FIG. 1, which illustrates a schematic diagram of a prior art thin film transistor (TFT) LCD device 10. The LCD device 10 includes an LCD panel 122, a timing controller 102, a source driver 104, and a gate driver 106. The LCD panel 122 is constructed by two parallel substrates, and the liquid crystal molecules are filled up between these two substrates. A plurality of data lines 110, a plurality of scan lines 112 that are perpendicular to the data lines 110, and a plurality of TFTs 114 are positioned on one of the substrates. There is a common electrode installed on another substrate for outputting a common voltage Vcom via the common electrode. Please note that only four TFTs 114 are shown in FIG. 1 for simplicity of illustration. In actuality, the LCD panel 100 has one TFT 114 installed in each intersection of the data lines 110 and scan lines 112. In other words, the TFTs 114 are arranged in a matrix format on the LCD panel 122. The data lines 110 correspond to different columns, and the scan lines 112 correspond to different rows. The LCD device 10 uses a specific column and a specific row to locate the associated TFT 114 that corresponds to a pixel. In addition, the two parallel substrates of the LCD panel 122 filled up with liquid crystal molecules can be considered as an equivalent capacitor 116.
The operation of the prior art LCD device 10 is described as follows. First, the timing controller 102 generates data signals for image display as well as control signals and timing signals for driving the control panel 122. The source driver 104 and the gate driver 106 generate input signals for different data lines 110 and scan lines 112 according to the signals sent by the timing controller 102 for turning on the corresponding TFTs 114 and changing the alignment of liquid crystal molecules and light transmittance, so that a voltage difference can be maintained by the equivalent capacitors 116 and image data 122 can be displayed in the LCD panel 100. For example, the gate driver 106 outputs a pulse to the scan line 112 for turning on the TFT 114. Therefore, the voltage of the input signal generated by the source driver 104 is inputted into the equivalent capacitor 116 through the data line 110 and the TFT 114. The voltage difference kept by the equivalent capacitor 116 can then adjust a corresponding gray level of the related pixel through affecting the related alignment of liquid crystal molecules positioned between the two parallel substrates. In addition, the source driver 104 generates the input signals, and magnitude of each input signal inputted to the data line 110 corresponds to different gray levels.
If the LCD device 10 continuously uses a positive voltage to drive the liquid crystal molecules, the liquid crystal molecules will not quickly change a corresponding alignment according to the applied voltages. Similarly, if the LCD device 10 continuously uses a negative voltage to drive the liquid crystal molecules, the liquid crystal molecules will not quickly change a corresponding alignment according to the applied voltages. Thus, the incident light will not produce accurate polarization or refraction, and the quality of images displayed on the LCD device 10 deteriorates. In order to protect the liquid crystal molecules from being irregular, the LCD device 10 must alternately use positive and negative voltages to drive the liquid crystal molecules. In addition, not only does the LCD panel 122 have the equivalent capacitors 116, but the related circuit will also have some parasitic capacitors owing to its intrinsic structure. When the same image is displayed on the LCD panel 100 for a long time, the parasite capacitors will be charged to generate a residual image effect. The residual image with regard to the parasitic capacitors will further distort the following images displayed on the same LCD panel 122. Therefore, the LCD device 10 must alternately use the positive and the negative voltages to drive the liquid crystal molecules for eliminating the undesired residual image effect, for example column inversion and dot inversion schemes are exploited.
Please refer to FIG. 2 and FIG. 3. FIG. 2 and FIG. 3 are schematic diagrams of a prior art column inversion driving approach. Blocks 20, 30 show polarities of pixels in the same part of two successive image frames. Comparing the blocks 20 and 30, when the LCD panel 122 is driven by the column inversion driving method, polarities of pixels in each column are identical and change to opposite polarities as a frame changes. Furthermore, polarities of pixels in two adjacent columns are opposite.
Apart from the driving approach mentioned above, the prior art can drive the LCD panel 122 in another way. Please refer to FIG. 4 and FIG. 5, which are schematic diagrams of a prior art dot inversion driving approach. Blocks 40, 50 show polarities of pixels in the same part of two successive image frames. Comparing the blocks 40 and 50, when the LCD panel 122 is driven by the dot inversion driving method, polarities of two adjacent pixels are opposite.
As mentioned above, when the driving voltages of the LCD panel 122 begin to reverse polarities, the LCD device 10 has the largest loading since the source driver 160 consumes the largest amount of current at this point in time. Generally, charge sharing is exploited to reuse electrical charges and reduce the reaction time that the equivalent capacitors 116 are charged to the expected voltage level. Further, power saving can be achieved. In the LCD device 10, the source driver 104 evenly allocates electrical charges by controlling transistor switches between two adjacent data lines to achieve charge sharing. Please refer to FIG. 6, which is a schematic diagram of voltage levels of an odd data channel and an even data channel next to the odd channel when an LCD is driven by the dot inversion driving approach according to the prior art. As shown in FIG. 6, the X-axis represents time and the Y-axis represents voltage level. The maximum and minimum driving voltage outputted to the equivalent capacitors 116 can be represented by VDD and VGND. The voltage level after charge sharing can be represented by Vavg. If the liquid crystal molecules are driven in the positive polarity, driving voltage Vp output to the equivalent capacitors 116 must be between the common voltage and the maximum driving voltage VDD. If the liquid crystal molecules are driven in the negative polarity, the driving voltage Vp output to the equivalent capacitors 116 must be between the minimum driving voltage VGND and the common voltage.
If the LCD panel 122 of the LCD device 10 is driven by the dot inversion driving approach, as shown in FIG. 6, when a driving period ends, the voltage level of the equivalent capacitor of an odd data channel CH_ODD is equal to the maximum driving voltage VDD, and the voltage level of the equivalent capacitor 116 of an even data channel CH_EVEN is equal to the minimum driving voltage VGND, assuming Vcom=0.5 VDD, and VGND=0. Before the next driving period starts, the LCD device 10 in the prior art first turns on transistor switches coupled to two adjacent data channels to perform charge sharing and neutralize electrical charges stored in liquid crystal capacitors in the end of the driving period. Thus, the voltage level of the equivalent capacitor of the odd data channel CH_ODD is pulled from Vp to Vavg. Similarly, the voltage level of the equivalent capacitor of the even data channel CH_EVEN is pulled from Vn to Vavg. Assuming Vp and Vn are equal to the maximum and minimum driving voltage, respectively, Vag=Vcom=0.5 VDD. During the next driving period, the polarity of the odd data channel CH_ODD turns from positive to negative. Since the source driver 102 discharges the odd data channel CH_ODD in advance through charge sharing, only a voltage difference ΔV=−0.5 VDD is provided for driving the liquid crystal molecules to control the gray levels of the relative pixels. Similarly, during the next driving period, the polarity of the even data channel CH_EVEN turns from negative to positive. Since the source driver 102 charges the even data channel CH_EVEN in advance through charge sharing, only a voltage difference ΔV=−0.5 VDD is provided for driving the liquid crystal molecules to control the gray levels of the relative pixels.
However, according to the prior art, the pixels in the same column and the same frame have identical polarities in the column inversion driving approach. Therefore, the performance of charge sharing discharges the electrical charges and turns polarity from positive to negative. Consequently, more power consumption will be caused if the polarity must remain positive. Please refer to FIG. 7, which is a schematic diagram of voltage levels of an odd data channel and an even data channel next to the odd channel when an LCD is driven by the column inversion driving approach according to the prior art. In FIG. 7, the X-axis represents time and the Y-axis represents voltage level. When a driving period ends, the voltage level of the equivalent capacitor of an odd data channel CH_ODD is equal to the maximum driving voltage VDD, and the voltage level of the equivalent capacitor of an even data channel CH_EVEN is equal to the minimum driving voltage VGND, assuming Vcom=0.5 VDD, and VGND=0. Before the next driving period starts, the LCD device 10 in the prior art first turns on transistor switches coupled to two adjacent data channels to perform charge sharing and neutralize electrical charges stored in liquid crystal capacitors in the end of the driving period. Thus, the voltage level of the equivalent capacitor in the odd data channel CH_ODD is pulled from Vp to Vavg. Similarly, the voltage level of the equivalent capacitor in the even data channel CH_EVEN is pulled from Vn to Vavg. In this situation, if the odd data channel CH_ODD intends to stay positive and the even data channel CH_EVEN intends to stay negative in the next driving period, the source driver 104 must provide an extra-absolute voltage difference |ΔV|=0.5 VDD for the displaying unit. In other words, charge sharing does not save power, but causes even greater power consumption.
As shown above, charge sharing cannot be adapted to all kinds of driving approaches according to the prior art; for example, in column inversion driving approach, extra power consumption may be caused.